1. Field of the Invention
The present invention relates to a cathode ray tube and, more particularly, to a color cathode ray tube equipped with an in-line electron gun having improved focusing characteristics.
2. Description of the Prior Art
A color cathode ray tube for displaying a color image in a TV receiver or a color monitor has a vacuum envelope composed of a panel portion serving as a picture screen, a neck portion for accommodating an electron gun, and a funnel portion connecting the panel portion and the neck portion. The funnel portion is equipped with a deflection yoke for scanning an electron beam emitted from the electron gun horizontally and vertically over a phosphor layer on the inner face of the panel portion.
FIG. 1 is a sectional view for explaining the entire structure of the color cathode ray tube of this kind. Reference numeral 1 designates a panel portion, numeral 2 a neck portion, numeral 3 a funnel portion, numeral 4 a phosphor layer, numeral 5 a shadow mask, numeral 6 a mask frame, numeral 7 a magnetic shield, numeral 8 a spring suspension mechanism, numeral 9 an electron gun, numeral 10 a deflection yoke, and numerals 11, 12 and 13 magnets for beam-centering corrections or color purity correction.
In the structure shown, the electron gun 9 accommodated in the neck portion 2 includes a cathode, a control electrode, focusing electrodes and accelerating electrodes. The electron gun 9 thus constructed modulates the electron beam from the cathode with video signals and causes the modulated electron beam to impinge upon the aforementioned phosphor layer 4 with a desired sectional shape and energy applied to it through the focusing electrodes and accelerating electrodes which constitute a main lens.
Electron beams Bc (center beam) and Bs (side beams) are horizontally and vertically deflected in their paths from the electron gun 9 to the phosphor layer 4 by the deflection yoke 10 mounted on the funnel portion 3.
FIG. 2 is a side elevation view for explaining the electrode construction of the electron gun accommodated in the neck portion of the color cathode ray tube shown in FIG. 1. Reference numeral 91 designates a first grid electrode, numeral 92 a second grid electrode, numeral 93 a third grid electrode, numeral 94 a fourth grid electrode, numeral 95 a shield cup, numeral 96 a bead glass, numeral 97 a stem, and letter K a cathode electrode.
As shown, the electron gun is constructed by arranging the aforementioned cathode electrode K, first grid electrode 91, second grid electrode 92, third grid electrode 93 and fourth grid electrode 94 in the recited order and embedding these electrodes fixedly in the bead glass 96 made of an insulating glass material with predetermined spacings between the electrodes and these individual electrodes are impressed with voltages through the stem 97 or a contact spring which is mounted in the shield cup 95.
FIG. 3 is a sectional view showing an essential portion for explaining the electrode structure of the main lens portion of the aforementioned electron gun in more detail. The same reference numerals as those of FIG. 2 designate the identical portions. Reference numeral 93-1 designates an aperture electrode disposed in the third grid electrode 93 (i.e., a beam-entrance-side electrode of the main lens), and numeral 94-1 designates an aperture electrode which is disposed in the fourth grid electrode 94 (i.e., a beam-exit-side electrode of the main lens). The opposing electrodes constituting the main lens portion (i.e., the third grid electrode 93 and the fourth grid electrode 94) are made of cylindrical electrodes having a racetrack-shaped section with its major axis being in the inline direction of the three electron beams.
FIG. 4 is an enlarged sectional view of the fourth grid electrode, and FIG. 5 is an enlarged front elevation view showing the fourth grid electrode. Reference numeral 94a designates an aperture for the center electron beam, numeral 94b apertures for the side electron beams, numeral 94-2 edges of the side electron beam apertures, and numeral 94-3 edges of the center electron beam aperture (as shown in U.S. Pat. No. 4,599,534, for example).
As shown, the fourth grid electrode 94 of the prior art is made of a cylindrical electrode having a racetrack-shaped section so that it is rotationally asymmetric (i.e., non-circular). This results in a difference in the focusing characteristics between the center electron beam and the side electron beams.
If the electrodes of the main lens portion are thus rotationally asymmetric, the main lens for the center electron beam and the main lens for the side electron beams fail to have equal characteristics. For example, the focusing voltage for the side electron beams becomes lower than that for the center electron beam. If the focusing voltage is adjusted for optimum focus of the side electron beams, the spot diameter of the center electron beam becomes larger than that of the side electron beams.
In order to obtain equal characteristics in the main lenses for the center electron beam and for the side electron beams, the focusing characteristics are improved by optimizing the shapes of the apertures of the aperture electrodes 93-1 and 94-1 inserted in the cup-shaped electrode constituting the main lens portion shown in FIG. 3.
The shapes of the apertures of the aperture electrodes 93-1 and 94-2 can be designed independently of each other for the center electron beam and the side electron beams so that the electric fields of the main lenses for the center electron beam and the side electron beams can be controlled substantially independently of each other.
In FIG. 3, the spacing (i.e., dimension S) between the electron beams is usually about 5.5 mm. As a result, it is difficult to make the focusing characteristics of the main lens for the center electron beam match those of the main lens for the side electron beams and to retain keeping the overall focusing characteristics of the main lenses sufficiently at the same time.